Glow plugs, which are used in assisting a start of an internal combustion engine, such as a diesel engine, include a tubular housing and a heater member which is energized to be heated. The heater member may be a ceramic heater having a heating element made of a conductive ceramic or a sheathed heater having a heating coil.
The housing has a screw portion dimensioned to be screwed into a mounting hole of the internal combustion engine and a tool engagement portion dimensioned to engage a tool when said housing is mounted to the internal combustion engine. An outer circumference of the tool engagement portion has a shape, e.g., a hexagonal shape in cross section, which allows the engagement of a tool, such as a wrench, whereby when the glow plug is mounted to the internal combustion engine, a force is applied along a circumferential direction of the tool engagement portion.
Further, an inner circumferential portion of such a tool engagement portion is generally formed into a circular shape in cross section by a cutting process, a forging process or the like. In a cutting process, a lathing operation is effected by means of a drill, a cutting bit or the like, and in a forging process, a round rod-like core metal is pushed into a rear end portion of the tool engagement portion (refer to, for example, JP-A-2010-210102).
Since the circumferential force is applied to the tool engagement portion when the glow plug is mounted to the internal combustion engine, the tool engagement portion needs a mechanical strength by which the tool engagement portion is able to withstand the applied circumferential force. Because of this, the thickness of the tool engagement portion should be equal to or larger than a predetermined value.
When the outer circumference of the tool engagement portion is formed into the hexagonal shape in cross section and the inner circumference is formed into the circular shape in cross section, in the event that the thickness of the thinnest portion of the tool engagement portion is set to the predetermined value, portions other than the thinnest portion of the tool engagement portion will be formed thicker than required. Consequently, the weight of the tool engagement portion and hence the weight of the housing becomes relatively large. As a result, there is concern that the fuel economy of the vehicle is deteriorated or the manufacturing cost of the glow plug is increased.
The invention has been made in view of the situations described above. An object of the present invention is to provide a glow plug which can realize an improvement in fuel economy or a reduction in manufacturing cost by reducing the weight of a tool engagement portion and hence the weight of a housing thereof, and a method for manufacturing the same.